The present invention relates to the field of lithium-ion batteries and, more particularly, to state-of-the art lithium-ion negative electrodes for lithium-ion batteries.
Due to the exponential growth in global energy consumption, rapid depletion of fossil fuels, concomitant growth in greenhouse gas emissions, and the upward spike in the prices of crude-oil and gasoline, significant concerns and efforts have been focused on the development of clean and renewable energy sources and advanced energy storage technologies.
Lithium-ion batteries are a type of rechargeable battery in which lithium ions move between the negative and positive electrode. The lithium ion moves through an electrolyte from the negative to the positive during discharge, and in reverse, from the positive to the negative, during recharge. Most commonly the negative electrode is made of graphite, which material is particularly preferred due to its stability during charge and discharge cycles as it forms solid electrolyte interface (SEI) layers with very small volume change.
Lithium ion batteries are finding ever increasing acceptance as power sources for portable electronics such as mobile phones and laptop computers that require high energy density and long lifetime. Such batteries are also finding application as power sources for automobiles, where recharge cycle capability and energy density are key requirements. In this regard, research is being conducted in the area of improved electrolytes, and improved electrodes. High-capacity electrodes for lithium-ion batteries have yet to be developed in order to meet the 40-mile plug-in hybrid electric vehicle (HEV) energy density needs that are currently targeted.
Further development of high-performance rechargeable lithium-ion batteries (LIBs) is indispensable for the ever growing needs for electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). Remarkable research efforts have been devoted to improving the already incomparable performance of rechargeable LIBs which are ubiquitous in various fields since its successful commercialization about 20 years before.
The most popular graphite-based anodes, with a theoretical specific capacity of 372 mAh/g, are commonly used in commercially available rechargeable LIBs along with several types of Li oxide based cathodes (less than 170 mAh/g specific capacity). However, current graphite and transition metal oxide based electrodes only provide moderate energy-storage capability and therefore it is difficult for them to meet the increasing demands for advanced energy storage. Hence, it is essential to design and synthesize new anode materials that can offer the promise of high-performance LIBs with elevated efficiency, superior storage capacity and gravimetric energy density, longer cycle life, easier state-of-charge control, lower cost, and safer operation.
Silicon-based electrodes for rechargeable LIBs have attracted considerable attention because they are able to vastly improve the specific capacity of batteries. As a naturally abundant element, silicon has the highest theoretical specific capacity among all exiting materials, which can reach 4200 mAh g−1 in the form of Li4.4Si. Furthermore, Si is also inexpensive, easy to handle, and has low discharge potential when used as an anode for rechargeable LIBs. These unique attributes endow Si as one of the most promising candidates to replace graphite as the anode for high performance rechargeable LIBs.
Therefore, one approach is to replace graphite as the negative electrode with silicon. Notably graphite electrodes are rated at 372 mAh/g (milliamp hours per gram) at LiC6, while silicon electrodes are rated more than tenfold better at 4,200 mAh/g at Li4.4Si. However, numerous issues prevent this material from being used as a negative electrode material in lithium-ion batteries. Full capacity cycling of Si results in significant capacity fade due to a large volume change during Li insertion (lithiation) and removal (de-lithiation). This volumetric change during reasonable cycling rates induces significant amounts of stress in micron size Si particles, causing the particles to fracture. Thus an electrode made with micron-size Si particles has to be cycled in a limited voltage range to minimize volume change.
Decreasing the Si particle size to nanometer scale can be an effective means of accommodating the volume change. However, the repeated volume change during cycling can also lead to repositioning of the particles in the electrode matrix and result in particle dislocation from the conductive matrix. This dislocation of particles causes the rapid fade of the electrode capacity during cycling, even though the Si particles are not fractured. Novel nano-fabrication strategies have been used to address some of the issues seen in the Si electrode, with some degree of success.
Unfortunately, the potential of Si particles in broad commercial applications continues to be hindered by severe capacity fading and loss of electrical contact caused by huge volume change, structural crumbling, and even cracking during repeated charge and discharge cycling, especially at high current rates.
Downsizing from conventional bulk silicon to various nanoscale morphologies and structures or dispersing these nanostructured Si into carbon matrices are among the most appealing approaches being pursued to overcome these issues and to improve the overall electrochemical performance of Si-based anodes in rechargeable LIBs.
Thus, the size reduction can help to accommodate the volume change, release the huge stresses in the Si particles during continuous insertion/extraction processes, facilitate more efficient electronic/ionic diffusion, provide more active sites, and enhance structural flexibility as well, while the carbon component in the Si/carbon nanocomposite electrodes can create a conducting matrix to maintain the electrical contact of the electrode with the current collector, resulting in better endurance during charge/discharge cycling. In addition, the incorporation of Li-active Si into carbon-based electrodes can reduce the initial irreversible capacity, and improve both the Coulombic efficiency and cycling performance of anodes even at high current densities.
Graphene, a new class of two-dimensional, “aromatic,” monolayer of carbon atoms densely packed in a honeycomb crystal lattice, has attracted unmatched attention and has also triggered tremendous experimental activities for applications in next generation electronic and energy storage devices, owing to its exceptional properties including extraordinarily high electronic mobility, outstanding optical transparency, unique electronic structures, intriguing thermal conductivity, and amazing mechanical strength as well as ultrahigh surface area.
Hence, graphene could be superior to other carbon materials as a conductive matrix to enhance electron transport and electrical contact with Si active materials in rechargeable LIB s and to effectively prevent the volume expansion/shrinkage and aggregation of Si phases during the Li charge/discharge processes. Furthermore, its large surface area can also facilitate the absorption of Li atoms on both sides of the graphene sheet or into its ubiquitous cavities. As a result, the merits of both carbon and Si phases can be extended to the largest extent based on their synergetic effects.
Recently, Chou et al. (S.-L. Chou, J.-Z. Wang, M. Choucair, H.-K. Liu, J. A. Stride, S.-X. Dou, Electrochemistry Communications 2010, 12, 303) blended commercially available nanosized Si particles and graphene to prepare eco-friendly and low cost LIB anodes, which exhibited enhanced cycling stability. In the meantime, several other groups also successfully prepared Si nanoparticles/graphene paper composite as anodes for rechargeable LIBs with high Li storage capability and cycling stability. (See J. K. Lee, K. B. Smith, C. M. Hayner, H. H. Kung, Chemical Communications 2010, 46, 2025: G. Wang, B. Wang, X. Wang, J. Park, S. Dou, H. Ahn, K. Kim, Journal of Materials Chemistry 2009, 19, 8378).
The studies also indicated that graphene can be used to anchor electrochemically active transition metal oxides or metal nanoparticles as anode materials for rechargeable LIBs, and these batteries exhibit enhanced cycle life and improved reversible capacity. See, for example, US Published Patent Application 2011/0033746, filed Aug. 10, 2009. The use of Si nanoparticles, however, may not provide a simple way to optimize the ion transport in the anode, especially when the loading of Si is high. Furthermore, the use of inactive binders to hold the Si and Carbon components together is required, which serves to reduce the overall energy capacity.
However, all of the above processes incur significantly higher manufacturing costs, as some of the approaches are not compatible with current Li ion manufacture technology. Thus, there remains the need for a simple, efficient and cost effective means for improving the stability and cycle-ability of silicon electrodes for use in lithium ion batteries.